CRTs and liquid crystal displays are widely used as terminals for displaying images such as characters, still images, and video images. With these devices, digital data can be instantaneously displayed and rewritten. However, these devices are not easy to carry around all the time. Furthermore, as these devices are self-luminous devices, they are disadvantageous in that human eyes may become fatigued by usage over a long period of time. Another disadvantage is that the displayed image cannot be saved if the power is turned off. Meanwhile, in order to distribute the characters and still images or to save them as documents, they are recorded onto a paper medium with a printer. A paper medium is widely used for producing hard copies. With the use of a hard copy, a user views a reflection caused by multi-scattering. Therefore, a hard copy is more advantageous than viewing a self-luminous device in terms of visibility, and causes less fatigue. Furthermore, a hard copy is light-weight and can be conveniently handled, and therefore the user can read a hard copy in any posture. However, after a hard copy is used, it may be discarded. Some paper sheets can be recycled, but recycling paper requires a tremendous amount of work and cost, which are problematic in terms of saving resources. In recent years and continuing, with the development of information devices, information processing such as creating documents can be performed with a computer. Accordingly, there are increasingly more opportunities for reading documents displayed on a display terminal.
Under such circumstances, there are increased requirements for a paper-like display medium that has both advantages of a display device and a hard copy, that can rewrite the data, and that is suitable for reading. There is a display medium realized with macromolecular dispersion type liquid crystal, bistable cholesteric liquid crystal, an electrochromic device, or an electrophoretic image display element. Recently, this type of display medium has been attracting attention because it can display a bright image even though it is a reflection type medium, and also because it is capable of saving images in its memory. A display medium realized with an electrophoretic image display element is particularly advantageous in terms of power consumption while displaying images. For example, patent document 1 discloses an invention based on such a principle.
A display medium employing an electrophoretic method has dispersion liquid encapsulated inside a colored dispersion medium between a set of transparent electrodes. Plural electrophoretic particles having a color different from that of the dispersion medium are dispersed in the dispersion liquid. The surfaces of the electrophoretic particles are electrically charged in the dispersion medium. Thus, if a voltage that attracts the charges of the electrophoretic particles is applied to one of the transparent electrodes, the electrophoretic particles will be attracted to and accumulated on that transparent electrode, and the color of the electrophoretic particles can be viewed. If a voltage that repels the charges of the electrophoretic particles is applied to one of the transparent electrodes, the electrophoretic particles will move toward the opposite transparent electrode, and the color of the dispersion medium can be viewed. An image can be displayed by utilizing such changes.
An electrophoretic image display element including a display medium employing such an electrophoretic method is an individual image display element. In order to realize an image display device, it is necessary to have multiple electrophoretic image display elements arranged within a microscopic area. Accordingly, a structure for arranging these elements is required. A sheet having a honeycomb structure, which is an assembly of plural hollow bodies, is known as a preferable structure for image display elements (image display element structure) for sectionalizing and arranging such microscopic elements. Electrophoretic particles and a dispersion medium are put in each honeycomb compartment, so that each honeycomb compartment acts as an image element, and the entire honeycomb structure acts as an image display device.
For example, patent document 2 discloses an electrophoretic image display and a manufacturing method thereof. This electrophoretic image display has plural cup-shaped recessed portions formed by a micro-embossing process or by image exposure. Each recessed portion is filled with charged dye particles that become dispersed in a solvent or a solvent blend. An overcoat layer is formed on the dispersed substance by a curing process. The overcoat layer is made of a sealing composition having a lower relative density than the dispersed substance, which sealing composition is at least partially nonmiscible with the dispersed substance. Accordingly, the recessed portion is sealed in such a manner that the dispersed substance is confined inside the recessed portion.
With respect to a method of manufacturing the image display element structure, patent document 3 discloses a method of manufacturing an anti-adhesive material made of a biodegradable film having a honeycomb structure. This honeycomb structure is formed with a biodegradable polymer and phosphorus surfactant. Due to the effects of the surfactant, the honeycomb structure is prevented from adhering to a biological body. Furthermore, the film thickness of this honeycomb structure is approximately 13 μm.    Patent Document 1: Japanese Laid-Open Patent Application No. 2004-189487    Patent Document 2: Japanese Patent No. 3680996    Patent Document 3: International Publication 2004/148680 Pamphlet
When the image display structure is applied as a display unit matrix of an image display device such as an electrophoretic image display, in order to display an image with high reflectance and high contrast, it is preferable that the image display structure have a high opening ratio, i.e., the partitioning walls between hollow bodies in the image display structure are preferably thin, particularly the partitioning walls on the displaying side.
In patent document 2, the honeycomb structure is formed by a micro-embossing process or by image exposure. In the case of a micro-embossing process, a die with patterns formed beforehand is used to emboss a precursor layer of thermoplastic or a thermosetting substance, which is coated on a conductive film. Subsequently, the precursor layer is cured by radiation, cooling, solvent evaporation, or by another method, and the cured precursor layer is removed from the die. In order to reduce the thickness of the walls in this method, the recessed portions of the die (between two protruding portions) need to be extremely thin. Hence, the precursor may not sufficiently enter the recessed portions such that the shape is not precisely transferred. Furthermore, when the intensity of the precursor is insufficient, some of the precursor may remain in the recessed portions of the die when the precursor is removed. Accordingly, it is difficult to form a honeycomb structure with hollow bodies with this method. Patent document 2 does not specifically describe the thickness of the walls partitioning the recessed portions. However, according to studies conducted by the inventors of the present invention, it is considered that when the partitioning walls are as thin as 10 μm or less, it is difficult to form a structure having a height that is greater than or equal to five times the wall thickness with the above method.
When forming the structure by image exposure, a conductive film coated with a radiation curing layer undergoes image exposure. Subsequently, after the exposed region becomes cured, the non-exposed region is removed. With this method, radiation is irradiated through a mask, or finely channeled radiation is directly irradiated onto the radiation curing layer to render a pattern. However, because the radiation light wraps around or scatters in the radiation curing layer, it is difficult to create walls of high aspect ratios.
Accordingly, there is a need for a manufacturing method and a manufacturing apparatus for an image display element structure having excellent reflectance and contrast. Furthermore, there is a need for a manufacturing method and a manufacturing apparatus for an electrophoretic image display element having excellent reflectance and contrast, employing the manufacturing method and the manufacturing apparatus for the image display element structure.